If you are driving and you have an active connection, Waze will still track your average speed across segments on your drive and will still record transition times between segments. You do not have to have navigation active for the real time Traffic data to be collected, driving with Waze open is enough.

My understanding is that waze accounts for the time it takes for each turn on a route no matter what direction the turn is in. Waze certainly accounts for some left turns taking more time as evidenced by the complaints that arise when waze recommends a right/U or straight/U/right maneuver rather than a simple left turn.